1. Field of the Invention
This invention relates to semiconductor fabrication technologies, and more particularly, to a phase-shifting mask (PSM) for use in a photolithographic process in semi-conductor fabrications, which can facilitate the automatic generation of the mask patterns thereon and help increase the resolution and contrast of the pattern definition.
2. Description of Related Art
In semiconductor fabrications, photolithography is an important and indispensable technique which is used to transfer circuit layout patterns through a mask onto predefined locations on an IC chip. Many processes in semiconductor fabrications, such as etching and ion implantation, require the use of photolithography. In a photolithographic process, resolution and depth of focus (DOF) are two major factors to appraise the quality of the pattern definition. A high level of integration requires a high resolution of pattern definition since the feature size is very small. To increase the resolution, a laser source with a very short wavelength, such as a krypton (Kr) deep ultra-violet laser with a wavelength of 2,480 .ANG. (angstrom), is used as the exposure light in the photolithographic process. The use of a short-wavelength exposure light, however, will result in a shallow DOF. To allow high resolution and good DOF, a solution is to use the so-called phase-shifting mask (PSM).
Fundamentally, a phase-shifting mask is formed by adding phase shifter layers on a conventional mask that can cause interference to the light passing therethrough such that the resolution of the resultant pattern definition can be increased. One benefit of the phase-shifting mask is that it can increase the resolution of pattern definition without changing the wavelength of the exposure light.
In semiconductor fabrications, line-shaped elements, such as word lines, bit lines, and metal interconnects in a DRAM device, are defined by forming a line pattern on a mask. Positive photoresist is customarily used in the photolithographic process. In sub-micron or higher level of integrations where line width is extremely small, the alternating PSM is customarily used for the photolithographic process. A conventional type of alternating PSM is depicted in detail in the following with reference to FIGS. 1A-1C, wherein FIG. 1A is a schematic top view of the conventional alternating PSM; FIG. 1B is a schematic sectional view of the alternating PSM of FIG. 1 A cutting through the line I--I; and FIG. 1C is a graph, showing the distribution of the illumination over the wafer caused by the exposure light passing through the alternating PSM of FIG. 1A.
As shown in FIGS. 1A-1B, the conventional alternating PSM includes a quartz plate 16 and a plurality of opaque mask patterns 10 (which can be films of chromium) and shifter layers 12 (which can be films of the compound MoSi.sub.z O.sub.x N.sub.y). The exposure light can pass through both the shifter layers 12 and the unmasked areas, as designated by the reference numeral 14, in the mask. The light passing through the unmasked areas 14 will not be shifted in phase, while the light passing through the shifter layers 12 will be shifted in phase by 180.degree.. FIG. 1C shows the distribution of illumination over the wafer caused by the light passing through the altemating PSM of FIG. 1A. Since the shifter layers 12 are arranged beside the opaque mask patterns 10 in an alternating manner over the phase-shifting mask, the diffractions of the light passing therethrough can be canceled out, thereby resulting in a zero-intensity point that can help increase the resolution of pattern definition on the wafer.
In the foregoing alternating PSM, the layout of the opaque mask patterns 10 and the shifter layers 12 can be generated automatically through computer graphic means. However, if the mask contains irregular or nonlinear patterns, they are not auto generatable through the computer graphic means. For example, in the case of FIG. 1A, the portion indicated by the reference numeral 18, due to its irregularly arranged manner, can not be recognized by the computer for automatic pattern generation.